1. Field of the Invention;
The present invention relates to a video camera that provides an auto focus function not only in a normal region ranging from telephoto to wide angle settings, but also in a macro region.
2. Description of the Prior Art;
Prior art video cameras having an auto focus function automatically drive the focus lens in what is known as a normal region between two extreme settings, i.e., between telephoto and wide-angle settings.
One known focus lens drive control method (as disclosed in Japanese Patent Application No. 62-146628) is based illustratively on the principle that the frequency components of the video signal, with the exception of the DC component thereof, are maximized at a point where the lens is in focus. Under this method, the frequency components of the video signal excluding the DC component thereof are accumulated, and the result is used as evaluation data. In operation, the lens is positioned so that the evaluation data is maximized. This is the so-called mountain climbing control. The focus ring for the lens is driven by a motor arrangement controlled in speed by varying the supplied DC voltage.
Assume that the focus lens, i.e., its focus ring, is controlled in movement relative to an object within a region between a near position N (wide angle) and a far position .infin. (telephoto) so that the curve of evaluation data Dt is obtained as shown in FIG. 9. In that case, if the focus lens is at a point P.sub.1 at the start of an auto focus operation, the focus lens is moved in a given direction so as to verify the direction in which the evaluation data Dt is maximized. As the focus lens is driven in that data maximizing direction, the evaluation data Dt is consecutively acquired. Near the peak of the evaluation data curve, the focus lens is fine-tuned in movement toward either the N or the .infin. setting. Ultimately, the focus lens is positioned at a point P.sub.j where the maximum evaluation data Dt is obtained.
The direction in which the auto focus operation starts, i.e., the direction in which the direction determining operation starts, is illustratively set based on the focus position derived from the immediately preceding auto focus operation. That is, the initial direction of focus lens movement is not predetermined to be either in the N or in the .infin. direction.
One disadvantage of the above-described auto focus control method is as follows: Assume that when the evaluation data Dt with its curve depicted in FIG. 10 is obtained regarding a given object, the focus lens is positioned at point P.sub.1 and that the focus lens is driven in the .infin. direction for direction determination at the start of an auto focus operation. In such a case, the appropriate evaluation data cannot be acquired during the direction determining operation. That is, when the lens is driven from a point P.sub.1 to a point P.sub.2 (i.e., a maximum telephoto position), the evaluation data Dt remains fairly constant at a very low level or is hardly obtained. This makes it impossible from point P.sub.1 to determine in which direction (toward .infin. or N) the peak of the evaluation data Dt is located. Thus the focus lens is driven up to the extreme point P.sub.2, where the lens changes its direction and starts moving in the N direction. It is only after the lens passes the point P.sub.1 that the appropriate evaluation data Dt is acquired. Thereafter, point P.sub.j is reached under mountain climbing control. That is, there occurs quite a redundant operation between points P.sub.1 and P.sub.2.
This is especially the case when the just-focus position is near either of the two extreme lens positions. If the focus lens starts being driven in the opposite direction of the just-focus position at the start of an auto focus operation, the focusing is inefficient and takes time. Furthermore, the sight of the focus lens moving in a redundant manner looks a little unseemly.
Some other disadvantages of the prior art will be mentioned below. The curve of the accumulated data shown in FIG. 9 necessarily varies in peak position and shape depending on the distance to the object and on the nature thereof. Thus, in an auto focus operation, the distance to the object always varies and the peak of the accumulated data curve varies accordingly. The varying peak position leads to focusing error.
Assume that when the object whose accumulated data curve is shown in FIG. 9 is being photographed, a second object such as a person or a car moves across between the object and the video camera. The evaluation data is accumulated with respect to the second object at the very moment the latter hides the original object when passing in front of the video camera. At that moment, the lens is automatically moved for focusing from point P.sub.1 to point P.sub.2. At point P.sub.2, the accumulated data drops abruptly as illustrated in FIG. 11. A drop in accumulated data under mountain climbing control causes the auto focus controller of the video camera to conclude that the lens position has gone past the peak of the accumulated data. Reversing the direction of lens movement, the auto focus controller erroneously judges that point P.sub.x immediately preceding the drop in the accumulated data is the just-focus position.
This is also the case where, during an auto focus operation with respect to a nearby object, the video camera is abruptly panned to take pictures of far-off sceneries. Assume that the video camera is panned while an auto focus operation is being carried out under mountain climbing control from point P.sub.1 to point P.sub.j(1) (i.e., a just-focus point) in accordance with the accumulated data curve shown by a solid line in FIG. 12. In that case, the broken line curve of the accumulated data is obtained starting from point P.sub.2, where the video camera was panned, in the .infin. direction. The accumulated data drops suddenly at the point P.sub.2, and the auto focus controller of the video camera erroneously concludes that the point P.sub.x is the peak of the curve. The same error also occurs when the video camera is abruptly panned from a far-off object to a nearby object.
The trouble above is summed up as follows: When a sudden change occurs to the current object during an auto focus operation under mountain climbing control, an erroneous judgment of the situation by the auto focus system makes it impossible to move the focus lens to the appropriate focal point. Thus, the photographing of high quality is not available.
To effect auto focus control requires detecting the focus lens position. If the focus lens is to be controlled in conjunction with a zoom lens, it is also necessary to detect the zoom lens position.
One prior art detection method for the focus lens position involves attaching a reflective film arrangement to a lens-mounted cylinder and providing a detecting means comprising light-emitting and light-receiving diodes, the reflective film arrangement giving reflected light whereby the lens position is detected. A method for effectively detecting the maximum positions of the lens is proposed by Japanese Patent Application No. 63-51293. The zoom lens is conventionally positioned in a simple manner illustratively involving the use of a resistor switch arrangement.
Auto focus control may be desired not only over the normal region but also over the macro region, i.e., ranging from the widest-angle position to the shortest-distance position of the normal region. This requires driving the zoom lens under the same mountain climbing control as for the focus lens. In that case, it is necessary to detect zoom lens positions with precision. For example, as depicted in FIG. 16, it is indispensable to detect various positions of the zoom lens, such as in telephoto T, middle telephoto MT, middle wide MW, or wide W intervals in the normal region, as well as the macro region area which includes at least two macro edge positions MCa and MCb of the zoom lens at the edges of the macro region and ma macro region MC in between them. A disadvantage is experienced with the prior art where seven zoom lens positions from telephoto T to macro region edge position MCb are to be detected, the disadvantage being the need to provide a large number of switching means for on/off control over the zoom lens positions within the respective regions. The numerous switching means tend to constitute a large-scale detecting mechanism that is difficult to incorporate in a compact video camera.
Furthermore, to drive the zoom lens requires relatively high levels of torque. Because the driving torque varies significantly from one driving unit to another, simply raising or lowering the DC voltage supply to the zoom ring motor is not sufficient to fine-tune the zoom lens movement while the lens movement speed is being adjusted. Where it is desired to move the zoom lens for a very short distance by lowering the DC voltage, one of two things may occur: the coefficient of mechanical static friction in the lens moving mechanism may be high enough to prevent the lens from moving at all, or the lens may move suddenly--and unpredictably--when the rising voltage reaches a certain critical level.
The above impediment makes it impossible to improve the means of the video camera for searching for the just-focus point in a narrow area in a fine-tuning manner during an auto focus operation within the macro region.